Rhodospirillum rubrum is a facultative microorganism that produces intracytoplasmic membrane during anaerobic growth in light or dark conditions. The membranes in light-grown cells contain pigment and cytochromes, and function to transduce radiant energy to form ATP. Although anaerobic dark-grown R. rubrum also forms pigmented membranes, the cells do not exhibit a light-stimulated cytochrome c2 oxidation-reduction reaction nor appear to transduce light energy. Both activities are acquired by dark-grown cells only after exposure to light. It is suggested that during the light-induced development process, either "rudimentary" membranes in dark-grown R. rubrum become functionally differentiated by insertion or modification of protein components; or new membrane is formed. To resolve alternatives, development of membrane function and membrane synthesis will be examined. The rate of development of membrane function and membrane synthesis will be examined. The rate of development of the c2 oxidation-reduction reaction will be measured spectrophotometrically and should correlate with the appearance of light-stimulated 32Pi-uptake during production of ATP. Membrane synthesis in anaerobic dark-grown cells exposed to controlled light conditions will be estimated by 14C-2-acetate fixation. Development of light-stimulated c2 oxidation in either existing or newly formed intracytoplasmic membrane will be visualized cytochemically using 3,3'-diaminobenzidine. The novel system of membrane development in R. rubrum provides an opportunity to study mechanisms which operate to differentiate rudimentary membranes into functionally specialized structures. These events may parallel membrane changes during cellular transformation of eucaryotic cells "triggered" by oncogenic agents.